The prior art patent literature shows several methods of controlling and limiting countersink depths of drilling machines. U.S. Pat. Nos. 2,574,653 (Miller) and 3,555,964 (Fleming) in particular employ threaded hard stops which are manually adjusted to limit the stroke of the drill feed systems. U.S. Pat. No. 4,717,291 (Zafir) utilizes a threaded hard stop positioned by a motor driven spur gear and ring gear arrangement, and is automatically adjusted for each previously measured fastener. U.S. Pat. No. 4,272,215 (Davert et al) describes a remote controlled wedge system which is driven back and forth to vary the height of the countersink stop. As described above, countersink hard stops adjusted by threaded arrangements or motor driven wedges are well known in the art. However, none of the prior art is illustrating use of non-contact eddy current systems, similar to the present system, to limit or monitor the countersink depth systems. U.S. Pat. No. 3,694,637 (Edwin et al) describes an apparatus to monitor tool wear as a function of change in power frequency characteristics of the tool during use. The system utilizes a small general purpose computer and detector circuit for analysis and tool monitoring respectively. U.S. Pat. No. 3,841,149 (Edwin et al) describes a tool wear detector system which compares a reference value representing the broadband vibrational energy output of an unworn tool to a running broadband average of the tool being used.
The present system in contrast shows the utilization of rapidly deviating voltage swings as detected by the eddy current displacement transducer to indicate tool condition and wear.